1. Field of the Invention
The present invention relates to programmable controllers for operating industrial equipment, and more specifically to the processors which execute a user defined control program in such programmable controllers.
2. Description of Related Art
Programmable controllers are a common type of industrial computer for operating a wide variety of manufacturing equipment, such as assembly lines and machine tools, in accordance with a stored program. The program comprises a series of process control instructions which are read out and executed to examine the condition of selected sensing devices on the controlled equipment, and to energize or deenergize selected operating devices contingent upon the status of one or more of the examined sensing devices.
The state of many sensing and operating devices can be represented by a single bit of data which is manipulated by the control program instructions. Other devices, such as position sensors, provide multiple bits of data representing a condition of the equipment being controlled. For these latter devices, instructions are provided to manipulate bytes and words of data representing the state of the sensing and operating devices. Additional program instructions perform arithmetic operations, timing and counting functions, and complex statistical reporting operations. Such instructions have become quite standardized in the industry and are directly associated with the elements of a ladder logic diagram which is easily understood by process control engineers. Program panels such as those described in U.S. Pat. Nos. 3,798,612; 3,813,649 and 4,070,702 have been developed to assist the user in developing and editing ladder logic control programs comprised of such programmable controller instructions.
An elaborate manufacturing process often is broken down into a series of smaller tasks, each defined by a separate ladder logic control program. The overall process is defined by a function chart specifying the order in which these tasks are to be performed and thereby the execution sequence of the corresponding ladder logic control programs. The use of a function chart in a programmable controller is described in U.S. Pat. No. 4,742,443 entitled "Programmable Controller with Function Chart Interpreter."
As programmable controllers were applied to more complex manufacturing systems, multiple controllers were used to govern the operation of different portions of the system. In such applications, it becomes necessary for one programmable controller to exchange data regarding the manufacturing process with the other programmable controllers. Techniques have been developed to communicate data among the programmable controllers as well as with a host computer that supervises the entire manufacturing system. In previous control networks, the host computer provided parameters for the manufacturing process, but could not directly affect the operation of the production equipment. However, it has become desirable to control specific operating devices throughout the manufacturing system from a central location. Thus, programmable controllers are being required to perform more communication functions. Such functions can adversely affect the primary control operation by taking processor time away from the execution of the user control program.